1. Technical Field
The present invention relates to an acceleration sensor and to a manufacturing method thereof.
2. Background Art
FIG. 6 is, for example, a plan view showing a part of an acceleration detector of a monolithic capacitance type disclosed in the Japanese Patent Publication (unexamined) No. 178954/1996. In this conventional art, the acceleration detector 100 contains a differential capacitor 100A surrounded by a dotted line in the drawing. The differential capacitor 100A includes a pair of capacitors. A first capacitor is formed between two electrodes 101, 103, and a second capacitor is formed between two electrodes 102, 104. The electrodes 101, 102 are electrically common. The common electrodes 101, 102 are lifted from a silicon substrate and are moving electrodes movable in response to acceleration. The other electrodes 103, 104 are stationary fixed electrodes. These electrodes 101, 102, 103 and 104 are all made of a polysilicon material. When acceleration is applied to the substrate, the movable electrodes 101, 102 being the common electrodes will move in such a manner that capacitance of the second capacitor may increase and capacitance of the first capacitor may decrease. These two capacitors are connected to a signal regulation circuit wherein the differential capacitance is converted into a corresponding voltage. By this voltage value, acceleration is detected.
It is a recent trend that smaller and cheaper acceleration sensor has been increasingly required. In the conventional acceleration sensor as described above, however, the polysilicon material deposited onto the silicon substrate by plasma CVD method must be formed approximately ten times as thick as a polysilicon membrane used in ordinary LSI. Therefore, the conventional acceleration sensor takes an extremely long period for deposition in a plasma CVD apparatus, resulting in a problem of longer construction time and increasing cost. Moreover, since distances between the moving electrodes 101, 102 and the fixed electrodes 103, 104 bear a direct relation to increase or decrease of capacitance of the capacitors, it is important to form these electrodes precisely at their positions. However, particularly the moving electrodes 101, 102 are liable to be warped due to residual stress of polysilicon, whereby the fixed electrodes 103, 104 and the moving electrodes 101, 102 may be varied from designed values in relative positions and distances thereof. Accordingly, performance of individual acceleration sensors is not always uniformly or stably exhibited thereby eventually causing another problem of lower product reliability.
The present invention was made to solve the above-discussed problems, and has an object of providing an acceleration sensor capable of being manufactured economically as well as easily and which is highly reliable. The invention also provides a manufacturing method of the acceleration sensor.
An acceleration sensor according to the invention comprises: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member disposed over the substrate surface and joined to said movable electrode to be displaceable together with said movable electrode;
wherein said mass member includes a thin polyimide membrane provided over the substrate surface and silicon nitride membranes provided respectively on a pair of main surfaces of said thin polyimide membrane substantially parallel to the substrate surface.
By this acceleration sensor according to the invention, a shorter construction time and lower cost have been achieved as compared with the conventional construction mainly fabricated of a polysilicon membrane. Further, since the silicon nitride membrane is provided as a flattening membrane onto a pair of the main surfaces of the thin polyimide membrane substantially parallel to the substrate surface, the thin polyimide membrane is restrained from being warped. Therefore the movable electrode and the fixed electrode can be formed at precisely the same relative positions and distances as designed. As a result, instability or non-uniformity in performance of individual apparatus can be surpassed whereby a highly reliable acceleration sensor can be eventually obtained.
In the acceleration sensor according to the invention, it is preferable that each of said silicon nitride membranes is covered with a metal membrane, and that end faces between the pair of said main surfaces of said thin polyimide membrane are covered with a metal membrane.
Another acceleration sensor according to the invention comprises: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member including a thin polyimide membrane disposed over the substrate surface and joined to said movable electrode to be displaceable together with the movable electrode,;
wherein a pair of main surfaces of said thin polyimide membrane substantially parallel to the substrate surface and end faces between the pair of the main surfaces of said polyimide membrane are respectively covered with a metal membrane.
By this acceleration sensor according to the invention, a shorter construction time, lower cost and higher reliability due to restraint of the polyimide membrane from being warped, a plating process conventionally required can be omitted. As a result, there arises a further advantage of simplifying the manufacturing process.
In the acceleration sensor according to the invention, it is preferable that said metal membrane is formed by the material selected from the group consisting of tungsten (W) and titanium nitride (TiN).
By this acceleration sensor according to the invention, the metal membrane is formed by the material selected from the group consisting of tungsten (W) and titanium nitride (TiN), thereby being capable of serving as a flattening membrane for restraining the polyimide membrane from being warped and as a metal membrane necessary for use as an electrode.
A method of manufacturing an acceleration sensor according to the invention, the acceleration sensor comprising: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member disposed over the substrate surface and joined to said movable electrode to be displaceable together with said movable electrode;
the method comprises the steps of:
forming a silicon oxide membrane on the substrate surface by plasma CVD method;
applying a first resist onto the silicon oxide membrane and patterning said first resist by photomechanical process;
removing a predetermined portion of said silicon oxide membrane by etching using said first resist as a mask;
forming a first silicon nitride membrane on the substrate surface by plasma CVD method for covering said silicone oxide membrane;
applying a polyimide material onto said first silicon nitride membrane and forming a thin polyimide membrane by setting the polyimide material at 300xc2x0 C. to 370xc2x0 C.;
forming a second silicon nitride membrane on said thin polyimide membrane by plasma CVD method;
applying a second resist onto said second silicon nitride membrane and patterning said second resist by photomechanical process;
etching sequentially said second silicon nitride membrane, said thin polyimide membrane and said first silicon nitride membrane using said second resist as a mask, thereby forming on the substrate surface a first block including the plurality of said first rod-like patterns at said predetermined portion, and a second block including the plurality of said second rod-like patterns and a mass block joined thereto;
removing said silicon oxide membrane by etching and separating said second block from the substrate surface; and
coating a main surface of said first block substantially parallel to the substrate surface and end faces thereof with a metal membrane thereby forming said fixed electrode, and coating a pair of main surfaces of said second block substantially parallel to the substrate surface and end faces thereof with the same metal membrane thereby forming said movable electrode and said mass member.
In this method of manufacturing an acceleration sensor according to the invention, the method includes the step of applying the polyimide material onto the first silicon nitride membrane and forming the thin polyimide membrane by setting the polyimide material at 300xc2x0 C. to 370xc2x0 C. Therefore the acceleration sensor can be manufactured in a shorter time and more easily as compared with the conventional method wherein the polysilicon membrane is formed by plasma CVD method. As a result, a shorter construction time and lower cost have been achieved.
Another method of manufacturing an acceleration sensor according to the invention, the acceleration sensor comprising: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member disposed over the substrate surface and joined to said movable electrode to be displaceable together with said movable electrode;
the method comprises the steps of:
forming a silicon oxide membrane onto the substrate surface by plasma CVD method;
applying a first resist onto said silicon oxide membrane and patterning said first resist by photomechanical process;
removing a predetermined portion of said silicon oxide membrane by etching using said first resist as a mask;
applying a polyimide membrane onto the substrate surface and forming a thin polyimide membrane by setting the polyimide material at 300xc2x0 C. to 370xc2x0 C.;
applying a second resist onto said thin polyimide membrane and patterning said second resist by photomechanical process;
etching said thin polyimide membrane using said second resist as the mask thereby forming on the substrate a first block including a plurality of first rod-like patterns at said predetermined portion, and a second block including a plurality of second rod-like patterns and a mass block joined thereto;
removing the mentioned silicon oxide membrane by etching and separating said second block from the substrate surface; and
coating a main surface of said first block substantially parallel to the substrate surface and end faces thereof with a metal membrane thereby forming said fixed electrode, and further coating a pair of main surfaces of said second block substantially parallel to the substrate surface and end faces thereof with the same metal membrane thereby forming said movable electrode and said mass member.
In the method of manufacturing an acceleration sensor according to the invention, the method includes the step of applying the polyimide material onto the first silicon nitride membrane and forming the thin polyimide membrane by setting the polyimide material at 300xc2x0 C. to 370xc2x0 C. Therefore the acceleration sensor can be manufactured in a shorter time and more easily as compared with the conventional method wherein the polysilicon membrane is formed by plasma CVD method. As a result, a shorter construction time and lower cost have been achieved.
In the method of manufacturing an acceleration sensor according to the invention, it is preferable that said metal membrane are formed the material selected from the group consisting of tungsten (W) and titanium nitride (TiN) and said metal membranes are formed by CVD method.
In the method of manufacturing an acceleration sensor according to the invention, it is preferable that said metal membrane are formed the material selected from the group consisting of tungsten (W) and titanium nitride (TiN) and said metal membranes are formed by sputtering method.